Will A Yamaha 25 Be Too Heavy For My Boat?

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Will A Yamaha 25 Be Too Heavy For My Boat?

Post by larski » 22 Aug 2016, 15:20

I have a 1977 Starcraft 16 foot long, 5 foot 2 inch beam "V"-shaped aluminum utility and am thinking of purchasing a 25 hp tiller-controlled Yamaha for it.

My question is whether the 175 pound Yamaha will be too heavy for the boat, especially if operated alone with no human ballast to move forward, and if so what I can do about fixing any resultant trim issues.

The boat is in very good condition overall and in excellent structural condition. I am increasing height of transom with marine ply and 1/4" thick aluminum plate (see photos). I am sure that transom will be strong enough for 25hp, my question concerns the weight of the 25 hp Yamaha.

The boat weighs 350 lbs empty hull and I weigh 175lbs.

I have tried my long shaft Yamaha 9.9 -- weighs 90 lbs -- on this boat with only me in it. Once on a plane, the trim is fine with no porpoising but the hole shot has the bow up in the air even when trimmed all the way in. The 9.9 pushes the boat to about 20 mph in relatively smooth water.

Part of the issue is that the transom is relatively narrow at 4' 8" as compared with today's boats which are somewhat wider. Further, this is an empty hull with no decking to offset the weight of the outboard.

I'm thinking that perhaps increasing the negative trim with wedges (bringing the bottom of the outboard closer to the transom to create more stern lift) and some ballast in the bow (say, around 70 - 80 pounds) when operating the boat alone may substantially mitigate the unfavorable effects of the heavy outboard? Maybe even some static trim tabs to help counter the lifting of the bow during hard acceleration (would like to avoid this option)?

I'm gonna put 100 pounds or so of ballast in the stern and see how boat sits and runs with the 9.9 and experiment with counter-acting bow ballast to get a feel as to whether this may be doable.

Any opinions / suggestions in the meantime?
Transom Ext Wood.jpg
Transom Angle.jpg
Side View Boat.jpg
Inside of Boat.jpg
Back of Transom.jpg

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Will A Yamaha 25 Be Too Heavy For My Boat?

Post by DaleH » 22 Aug 2016, 16:40

Smart Tabs or SE Sport 200 fin would do it. Note if no hydraulic trim/tilt, you may find that you still need to manually adjust the trim pin, depending on load carried and WHERE it is carried.
#1) 1st tin rebuild, 18' Lund http://forum.tinboats.net/viewtopic.php?f=21&t=36583
#2) 25' Parker refurb from EMPTY hull http://www.classicparker.com/phpBB3/vie ... p?f=15&t=6
#3) 16' V-tin rebuild http://forum.tinboats.net/viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins http://forum.tinboats.net/viewtopic.php?f=3&t=37548

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Will A Yamaha 25 Be Too Heavy For My Boat?

Post by lckstckn2smknbrls » 22 Aug 2016, 20:00

The weight should be no problem. You may have to do some experimenting as to trim and load.
1971 14' Ward's Sea King Jon Boat / 1983 Mariner 25HP

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Will A Yamaha 25 Be Too Heavy For My Boat?

Post by turbotodd » 23 Aug 2016, 20:44

You'll be fine. You may try to put a little weight in the front. Maybe move the tank or battery to the front to offset some extra weight. Also the stock prop may or may not be optimal. Might pick up a tachometer and try it out. Usually on the newer hulls, we're seeing 6200 RPM roughly at 27 mph. They rate them at 6000 max. I've had good success using the old 2 stroke prop on the 4 strokes which picks up some speed and drops the RPM back to around 6000-ish without any measurable loss of hole shot.

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Will A Yamaha 25 Be Too Heavy For My Boat?

Post by Stubborn Dutchman » 24 Aug 2016, 07:46

I have a 16' DuraNautic with a 40 hp. Yamaha. It has p/t/t. I run solo most of the time. Even with the power trim I still experience some porpoising at full throttle. I'm going to add 40# of anchor chain and a trolling motor battery to the front. That should clear up my problem. I think your 25 hp should push your boat just fine.

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Will A Yamaha 25 Be Too Heavy For My Boat?

Post by Pappy » 24 Aug 2016, 10:39

Should be a great combination.....go for it!

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